Pumps with magnetically driven rotors that have an airgap along a spherical surface show significant advantages relative to other centrifugal pumps because the most unreliable elements of conventional centrifugal pumps, namely shafts, bushings and rotary sealings, are eliminated. The driven rotor, which forms a rotary unit with the impeller, is not only magnetically coupled with the driving system, but at the same time is axially supported by a spherical bearing that cannot block or seize.
Magnets of sufficient coercive force are made from barium or strontium ferrites or aluminium-nickel-cobalt alloys. Neither of these materials can be welded, brazed, or machined. Joining of the plastic impeller to the annular magnet presents a problem. While the outer driving magnet can be fixed by a suitable adhesive at the hub, this is not possible with the driven magnet because it is immersed in liquid, where adhesives would deteriorate even in water. Because magnetic material is unsuitable for machining, it has been necessary in the past to enclose ceramic magnets in a plastic housing. However, this method has the disadvantage that the thickness of the effective magnetic gap is increased by the thickness of the plastic housing, and this leads to a considerable decrease in the transmittable torque.